Nanomaterials have received continuous attention for their novel characteristics and large surface area induced by size reduction. Continued efforts have been made to prepare various nanosized materials and structures. The unique electrical and physical properties of nanomaterials are attributed to their structural features. Nanomaterials can also be used to change the characteristics of bulk materials through the formation of bulk composites based on their nanostructure. The characteristics of nanomaterials are difficult to achieve by general bulk materials that are at least composed of microscale grains as basic units. In contrast, individual nanoscale grains can change the electrical and physical properties of corresponding bulk materials. For better performance and various physical properties of general bulk materials, efforts have been made to modify the microstructure of the bulk materials. As representative examples, incorporation of dissimilar materials into base materials and phase separation are known. As another example, the use of recrystallization by annealing for grain size reduction is widely known to improve the mechanical properties of materials. However, such top-down approaches are only applied to microscale microstructures.
To overcome this limitation, attempts have been made to prepare bulk composites from nanopowders through sintering. The use of a bottom-up approach for preparing bulk composites from nanopowders by rapid sintering while minimizing the growth of grains enables the production of bulk materials having nanoscale grains. This approach avoids the need for additional annealing and processing and is thus economical in terms of cost and time compared to conventional methods. According to the Hall-Petch equation, the mechanical strength of a material is inversely proportional to the square root of the grain size. This explains improved mechanical properties of materials having nanoscale grains compared to existing materials. In addition, individual grains in nanomaterials enable control over the electrical properties of the materials due to their size effect. Furthermore, structural improvement of nanopowders is considered to control nanoscale microstructures beyond the limitation of existing microscale microstructures. Due to these advantages, research aimed at improving the performance of nanostructures through sintering of nanopowders has been conducted in various fields.
Lead chalcogenides are typical materials for thermoelectric generation and are energy materials for electricity production using waste heat that are dumped into the atmosphere. Thermoelectric generation has received attention as a next-generation technology for mutual conversion between thermal energy and electric energy. The development of existing thermoelectric generation technologies is dependent on the characteristics of materials. Thus, finding good materials for thermoelectric generation is considered important. As representative examples, Bi2Te3, PbTe, and SiGe are extensively used for thermoelectric generation. In recent years, high-performance thermoelectric materials have been developed through microstructural control. Under these circumstances, research has been conducted aimed at improving the characteristics of thermoelectric materials through microstructural control. Materials for high-performance thermoelectric generation are required to have high electrical conductivity and low thermal conductivity. According to the so-called Wiedemann Franz's law, electrons migrating with thermal energy in a material are known to be difficult to control independently. Since thermal conduction in a material is contributed by lattice vibrations (i.e., phonons) as well as electronic conduction, the total thermal conductivity of the material is lowered by reducing the lattice vibrations to induce maximum thermoelectric performance of the material, which is essential for high-performance thermoelectric technology. For this purpose, studies aimed at suppressing lattice vibrations through microstructural control are currently underway by Prof. Kanatzidis at Northwestern University (U.S.) and Prof. Snyder at California Institute of Technology (U.S.). According to previous studies on microstructural control, the grain size of bulk materials is reduced by crushing and secondary phases are precipitated through phase separation to suppress heat transfer by lattice vibrations, achieving improved thermoelectric performance. However, such top-down approaches are only applied to microscale microstructures, as mentioned above.
There is thus a need to develop a technique for nanoscale microstructure control through phase separation in a nanoscale size and a technique for the synthesis of a controlled nanopowder in the preparation of lead chalcogenides.